Elevator doors and similar closures are operated along tracks at variable speeds, so that the beginning and end of travel occur at slower speeds than the middle of travel. Typically, this is achieved using a variable speed motor and providing limit switches at extremes of door travel to turn the motor off.
There are various known ways of achieving a variable speed output for driving closures. Such mechanisms can be broadly grouped into four types: those employing meshing eccentric gears or cam surfaces; those utilizing stepped or variable diameter shafts; those employing eccentric sprockets or wheels for driving chains or belts; and other miscellaneous types.
U.S. Pat. Nos. 1,834,610 and 4,998,379 and Japanese Patent No. 144,392 show mechanisms for converting constant speed rotation into variable speed rotation utilizing meshing eccentric gears or cams. In the '610 patent, a driving gear eccentrically mounted on a drive shaft is rotated in mesh with a driven gear eccentrically mounted on a driven shaft, to rotate the drive shaft at varying speeds. At the start, door movement is slow, then accelerates, and is finally retarded again as the doors are moved into the closed position. A similar mechanism is employed in the Japanese patent. There, a non-circular drive gear is rotated by a constant velocity input shaft to drive a 180.degree. oppositely oriented similar non-circular driven gear, to periodically change the rotation speed of an output shaft between a maximum and a minimum. The '379 patent employs similar principles in a vehicle window regulator, wherein two elliptical gears are peripherally meshed to define a varying radius about an axis of rotation, so that a window travels at a faster speed near its fully closed position than when near its fully opened position.
Examples of variable speed drives employing stepped or variable diameter shafts are given in U.S. Pat. Nos. 3,043,584 and 5,046,283. In the '584 patent, a constant speed, continuously variable diameter cone is used to frictionally drive a rail which engages the cone at different diameters at different times during a door panel opening or closing procedure. A point at the periphery of the cone near the apex travels at measurably slower speed than a point near the base of the cone, so that the rail contacting the cone first at one and then at the other point will be propelled successively at slower, then higher, speeds. In a van door closing arrangement described in the '283 patent, for a motor turning at constant speed, the winding of a cable on a larger diameter portion of a reel causes a door to travel at a relatively high speed over large distance. Then, as the door approaches the closed position, the winding of the cable on a smaller diameter portion of the same reel causes the door to move at a slower speed, but with a greater force being applied to the cable, until the door is fully closed.
German Auslegeschrift 1,147,510 shows an arrangement for opening and closing doors that utilizes an eccentrically mounted input sprocket, rotating at constant velocity on an shaft, to drive a chain to turn a circular output sprocket at varying speeds. Slack in the chain due to varying radius of the input sprocket relative to the input shaft is taken up either by a spring-loaded auxiliary sprocket, or by another eccentric member, whose rotation is coordinated with the rotation of the input sprocket.
Other prior art arrangements for accomplishing door closure are given in U.S. Pat. Nos. 2,458,402 and 4,711,323.
Conventional elevator doors are suspended from movable sheave assemblies that roll along tracks attached to the cab, above the cab opening. Connection between the output of the drive motor and the sheaves is established either by cable connection or mechanical linkage. In a typical cable connection, a variable speed electric motor is mounted atop the cab and powered by a long flexible power line having one end connected to travel with the cab and the other end connected to a power control system fixed atop the hoistway. The sheave assemblies are connected to be drawn along the track by a cable that is driven by the motor and has a run disposed parallel with, and above, the track. For a single-slide door arrangement, one or both sheave assemblies are clamped to move the door from its closed to its open position, and back, at speeds and torques determined in compliance with applicable elevator codes (e.g. Elevator Code A17.1 or European Code EN 81). In a center-opening, two door arrangement, the driving run of cable usually takes the form of a loop of cable having two spaced, oppositely directed runs. The sheaves of one door are clamped to travel in one direction with one run and the sheaves of the other door are clamped to travel in the opposite direction with the other run. Two-speed slide door arrangements, having slow and fast doors arranged in pairs, connect in similar fashion, except that a traveling gearing bar assembly is interposed between the sheaves of each pair and the drive cable. The gearing bar assembly comprises a bar clamped to move with the main drive cable, and a secondary loop of cable which winds around opposite gearing pulleys and has one run clamped to the cab. The "fast" door is fixed to the bar, so travels with the bar and primary cable. The "slow" door, however, is fixed to the other run of the secondary loop, so travels at half the speed for half the distance.
Existing door tracks having C-shaped cross-sections provide inwardly-facing, upper and lower rails between which centrally grooved sheaves of the sheave assemblies are captured. Smaller eccentric rollers travel along the undersurface of the track to maintain stability. To hang the doors, the sheaves must be slid in from the ends of the track, which may be difficult to do in confined spaces. Further, because the sheaves of all sheave assemblies for each door ride on the same track, track lengths are difficult to fit for nonstandard cab or opening widths and must often be special ordered from the fabricator at customized sizes.